The invention relates generally to a cryogenic cooling system for high temperature superconductor (HTS) devices and more particularly to a cryogenic cooling system for FITS devices having high-voltage electric power applications.
There exists HTS cooling systems that use the properties of liquid nitrogen to achieve cryogenic cooling. Normally, liquid nitrogen is used at one atmospheric pressure (0.1 MPa) where its operating temperature (boiling point) is at 77 degrees Kelvin. However, since the critical current density of HTS materials improves significantly at temperatures lower than 77K, methods have been developed to reduce the temperature of the liquid nitrogen by manipulating its operating environment. FIG. 1 is a p(pressure)−T(temperature) diagram showing the relationship amongst the p, T and the three phases (solid, liquid and vapor/gas) of a typical substance. For nitrogen, the “Triple Point” is about 63.15K at 12.53 kPa. This shows by reducing the pressure of liquid nitrogen its boiling point temperature can be lowered to about 63K below which solid nitrogen would form. One example of using such properties of liquid nitrogen to achieve lower operating temperature is provided in U.S. Pat. No. 5,477,693. It describes a method of using vacuum pump to pump the gaseous nitrogen region in a cryogen containment vessel (cryostat) that contains both the liquid and gaseous nitrogen. Pumping reduces the pressure of the liquid nitrogen bath therefore reducing its temperature (boiling point) to below 77K. The performance of the superconductor, namely its critical current level, is then significantly improved.
Even though the prior art increases the performance of HTS materials by lowering the boiling temperature of liquid nitrogen through lowering its pressure, it is at the expense of significantly degrading the dielectric strength of liquid nitrogen and, as a consequence, such cooling systems are not suitable for high-voltage HTS applications. Typically, liquid cryogen based cooling systems for high-voltage HTS devices rely in large degree on the dielectric properties of the liquid cryogen as the main electrical insulation medium. There are two major factors that influence the dielectric properties of liquid nitrogen. One is the intrinsic dielectric strength of liquid nitrogen that is pressure dependent. FIG. 2 shows the dielectric strength of liquid nitrogen as a function of pressure. The strength decreases sharply when the pressure is below one atmospheric pressure (0.1 MPa) while the optimum value resides in the range of between 0.3 MPa and 0.5 MPa. The other major factor is the bubbles that occur in the liquid nitrogen. Bubbles, especially large size bubbles, tend to reduce the dielectric strength of liquid nitrogen. Bubbles will be generated when objects submerged in liquid nitrogen are heated to above the boiling temperature of liquid nitrogen. Lowering the boiling point in liquid nitrogen will thus make bubble generation more easily. Therefore the method of lowering liquid nitrogen temperature by lowering its pressure will have a negative impact on both factors that govern the dielectric strength of liquid nitrogen. Cooling systems based on such and similar approaches are therefore ill suited for high-voltage HTS applications.